A radioisotope thermoelectric generator (RTG) is a device that uses the heat generated by the natural decay of a radioactive isotope to produce electricity. It typically consists of an array of thermocouples connected to a heat source, such as a plutonium-238 or plutonium-239 pellet, and is used in spacecraft and other remote applications where solar power is not available. RTGs have been used in a variety of space missions, including the Voyager probes, Cassini–Huygens, Curiosity rover, New Horizons and Juno.